As I write this article, an estimate 31% of the expected Missouri corn acreage has been planted. This compares to 21 % in 2008 and 66% for the 5-year average. Frequent precipitation has provided few opportunities to work soil and/or plant corn.
Perhaps even more worrisome is that only 6% of the acreage has emerged and reports of emergence problems are quite common. Although weather conditions are similar to 2008, it appears that corn emergence problems are more severe and more widespread in 2009.
The causes of poor corn emergence can be difficult to assess and are highly dependent on soil conditions near the seed. Seed treatments can be beneficial under these less than optimum conditions. Unfortunately, a number of bad things, including things other than the usual suspects of diseases and insect pests, can happen to high quality seeds.
Newly planted corn seeds absorb water, and this increase in water content turns on all of the life processes of the seed. Unfortunately, water absorption itself can damage the seed if the water is cold. Cold water damages cell membranes, which surround the entire cell and all of the cell organelles. Damaged cell membranes can cause cell death, if the damage is severe. But, perhaps more problematic, is that weakened membranes allow cell contents to leak from the cell. Cell leakage of sugars and proteins is possible even if the membrane damage is not severe enough to cause cell death. These sugars and proteins act as a magnet for insects and pathogens. Cold water imbibition damage is difficult to diagnose because all the damage occurs below ground and the resulting cell leakage stimulates other problems that are more often considered the cause of emergence problems.
The first signs of germination are when the first root and the coleoptile emerge from the seed. The coleoptile is a leaf modified to appear like a tube. It is what we see when we say that corn is “spiking” and the first sign of emergence visible above ground. Both of these structures must rupture the seed coat to emerge. These ruptures tear openings in the seed that allow proteins and sugars to spill into the surrounding soil. These sugars and proteins are food sources for pathogens, and fungi can multiply rapidly as they feed on the leakage. Tears in the seed coat are entry points that allow pathogens and insects to invade the seed.
Life processes require abundant oxygen, so as seeds proceed through germination to emergence, oxygen demand rapidly increases. The amount of oxygen in the soil depends on the amount of pore space. Soil texture affects total pore space, and soil compaction decreases pore space. Water is stored in the same soil pore spaces as oxygen. More water in a soil means less oxygen. If oxygen supply is limited, actively growing seeds will deplete oxygen to the point where stress occurs. Limited oxygen causes cells to produce toxic compounds that may cause cell and seedling death.
All of these problems probably occur each year, but the effects on corn stands are usually minimal particularly when spring weather conditions improve after planting. The worse combination of conditions is fluctuating soil temperatures and frequent precipitation. Seeds are relatively “safe” if soil conditions prevent germination. The minimum temperature for corn seed germination is usually given as 50F, but slow germination will occur at temperatures in the high 40s. Dry soil does not contain enough water to stimulate germination. If the seed coat has not been damaged during handling, disease pathogens and even most insects will not damage seeds after planting into cold and dry soils.
However, once germination begins it is a race between seedling establishment and the seedling enemies. Fluctuating soil temperatures and water status allow germination to begin, but then slow the process to a point where the germinating seed and the emerging seedling are at a disadvantage.
Once seeds have been planted they are at the mercy of Mother Nature. Protection of the seed with effective seed treatments can help, but will not eliminate all problems leading to poor stands. Delayed planting may be in order, but not much help if soil conditions deteriorate after planting. Reducing activities that compact soil around seeds is important because compaction can reduce root health and delay emergence.
REVISED: October 2, 2015